140 Capt. Sabine’s observations on the magnetic needle. 
Observations to determine the variation of the needle , in Davis’s 
Strait, and Baffin’s Bay. 
The azimuth compasses used in these observations were 
constructed on an improved plan, the invention of Captain 
Kater. It is thus described in the “ Instructions for the use 
of the Instruments furnished to the Northern Expeditions,” 
printed by order of the Royal Society : 
“ The compass is five inches diameter ; by means of an 
inclined mirror and lenses, the degrees are seen by reflection 
considerably magnified ; a line drawn on a piece of ivory is 
viewed at the same time, and serves as an index by which 
the degrees are to be read off. 
“ At the opposite side of the box is a sight on which slides, 
in a frame, the segment of a glass cylinder, ground to a 
radius of five inches. By means of this, a fine line of light 
is thrown on the index, and may be seen at the same time as 
the degrees on the card. 
“ The degrees on the card are read from the north towards 
the east, and are carried round to 360°, in order to obviate 
the possibility of error in this respect.” 
The observations were made either on shore or on the ice, 
sufficiently distant from the ship to be beyond the influence 
of her attraction. The compass was placed on a copper 
fastened stool, and was carefully levelled by means of a spirit 
level, to ensure the perpendicularity of the sight vane. 
Each altitude and azimuth is a mean of several observa- 
tions, the compass being removed and levelled afresh between 
every one, thus making each faithfully distinct. 
